The present invention concerns a buoy. More particularly, but not exclusively, this invention concerns a tethered communications buoy for use with a submarine vessel. The invention also concerns a method of recovering a tethered buoy.
The integration of submarine vessels into naval surface operations has been limited by the lack of reliable high-bandwidth data communications systems. Such communications systems should preferably not affect the submarine's primary attribute of stealth, and should preferably be available at speed and depth and without limiting the maneuverability or performance of the submarine vessel.
Proposals for communication systems for a submarine include the use of a tethered communications buoy system. Some of those systems operate by deploying and recovering a communications buoy from a submerged submarine via a tether line, which may for example include a fibre optic cable. Towed buoy systems can generate surface wake potentially risking detection of the location of the submarine. A solution to this problem is to have different modes of operation including a surface mode in which the buoy floats in the water and communicates (in which mode the buoy is not towed by the submarine) and a second travelling mode in which the buoy is recovered by the submarine.
In a communications buoy system such as that described above, the buoy may be released from the submarine so that it rises to the surface under its own buoyancy, surfacing with low surface disturbance (wake or plume) to reduce the probability of detection. At the surface, the buoy may be engaged in communication during a “communications window”. During surface operations, the buoy is allowed to float on the surface whilst a tether line is continually deployed at very low tension from a winch on the submarine. When the communications window is concluded, the buoy system is recovered to the submarine by reversing the winch and reeling the buoy back in so that it travels beneath the water surface. Buoys suitable for this purpose are disclosed in WO 2005/120942, WO 2005/120943, and WO 2007/045864.
Communications buoys tend to suffer from various problems, some of which will now be described.
Buoys designed to operate directly below the surface suffer from low data rates. Thus, a communications buoy may include an antenna for receiving/transmitting data, the antenna being positioned at the upper end of the buoy so that the antenna is exposed and is above the water line when the buoy is floating at the water surface. However, in rough waters there may be interruptions to the communications window caused as a result of water washing over the antenna.
A buoy which is designed so that it floats in a stable and generally upright manner at the surface of the water generally has a shape and/or mass distribution such that it is not very well suited to travelling in a streamlined fashion through water. Also, it may be desirable for the buoy to travel at speed through the water without generating forces which cause the buoy to deviate from a desired path. For example, if the buoy has a tendency to rise in the water when being towed in a generally horizontal direction, the buoy might surface causing highly visible wake and plume on the water surface. On the other hand, if the buoy flies too low in the water (has a tendency to sink when towed at speed), the tether line used to tow the buoy may get too close to the propulsion unit at the rear of the submarine vessel. Various solutions have been proposed to these problems.
WO 2005/120492 describes a buoy having a stabilising tail and a yoke connected at one end to a tether line and pivotally connected at the other to the centre of buoyancy of the buoy. WO 2005/120943 discloses a buoy having a tail moveable between a closed position which minimises drag when the buoy travels through water and an open position in which the centre of mass is moved relative to the centre of buoyancy thus facilitating a stable floating configuration. The buoy of WO 2005/120943 also includes a pivotally moveable arm for lifting an antenna clear of the water. Both WO 2005/120942 and WO 2005/120943 have the disadvantage of having externally mounted moving parts of a complicated design and which might result in an undesirably large wake/plume at certain speeds/orientations of travel.
WO 2007/045864 discloses a buoy having fixed hydrodynamic surfaces for increasing the stability of the buoy when towed at certain speeds. The buoy must however travel at certain speeds to be stable in the water. The buoy has a tendency to rise (or sink) to varying degrees in the water, depending on the speed at which it is being towed in the horizontal direction.
The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved buoy.